On Earth, animal life expanded rapidly during the early Cambrian period, around 520 million years ago. Many theories have been advanced to explain this rapid expansion of animal species, which has come to be known as the Cambrian explosion.
Thursday, new research was published from two UK-based professors—Paul Smith, director of the Oxford University Museum of Natural History, and David Harper, professor of paleontology in the Department of Earth Sciences at Durham University—which points to a combination of interlinked factors being responsible for the rise of animals on Earth, rather than a single underlying cause.
Up till now, theories about why animal live should have burst forth during the Cambrian period in Earth’s distant past could be categorized into three groups: geological, geochemical and biological. Scientists have tended to look upon these as standalone explanations for animal life gaining a foothold on Earth.
The Cambrian explosion was a major evolutionary event evidenced by a diverse range of biological innovation. The basis of modern ecosystems can be traced back to the Cambrian period. As an example, predator species, such as the Anomalocaris, a forerunner of modern-day crabs and lobsters, once held sway at the top of the food chain. This free swimming, metre long proto-crustacean, thought to be an ancestor of today’s Arthropods—invertebrates with an exoskeleton—had a mouth composed of 32 overlapping plates that could constrict and crush prey.
But alongside such strange invertebrates, the Cambrian explosion also saw vertebrates making their first appearance. The ancient ancestors of today’s major families of vertebrates—fish, reptiles, birds and mammals—all started off life during the Cambrian period.
Just why the Cambrian period should be such a fertile period in Earth’s history for such zoological diversity has been examined by Smith, Harper and a team of scientists over a period of four years.
The base for their research was Sirius Passet in the very north of Greenland, overlooking the Arctic Ocean. Although not the most easily reached location for research, Sirius Passet, located at 83°N, just 500 miles from the North Pole, was chosen because of the high quality of the fossil material it’s known to provide.
The findings of Smith, Harper and their team, entitled, "Causes of the Cambrian Explosion," are published in the latest edition of the journal Science.
Lead author Smith said, “This is a period of time that has attracted a lot of attention because it’s when animals appear very abruptly in the fossil record, and in great diversity. Out of this event came nearly all of the major groups of animals that we recognise today.”
Smith added, “Because it is such a major biological event, it has attracted much opinion and speculation about its cause.”
In seeking to explain the Cambrian explosion, the researchers refer to a "cascade of events." A rise in sea levels early in the Cambrian period is the likely cause in kicking off the sequence. As sea levels rose, the area of inhabitable seafloor expanded. That, in turn, drove an increase in animal diversity.
These early events primed the pump for a complex interaction of biological, geochemical and geological processes, which individual hypotheses have focussed on up till now.
Harper, said, “The Cambrian explosion is one of the most important events in the history of life on our planet, establishing animals as the most visible part of the planet's marine ecosystems.”
He continued, “It would be naïve to think that any one cause ignited this phenomenal explosion of animal life. Rather, a chain reaction involving a number of biological and geological drivers kicked into gear, escalating the planet's diversity during a relatively short interval of deep time.”
He described what took place during the Cambrian explosion, setting the scene for much of the subsequent marine life, as building on cascading and nested feedback loops, linking organisms and their environment, some 520 million years ago.
Smith added, “Work at the Sirius Passet site in north Greenland has cemented our thinking that it wasn't a matter of saying one hypothesis is right and one is wrong. Rather than focusing on one single cause, we should be looking at the interaction of a number of different mechanisms.”
And he concluded, “'Most of the hypotheses have at least a kernel of truth, but each is insufficient to have been the single cause of the Cambrian explosion. What we need to do now is focus on the sequence of interconnected events and the way they related to each other – the initial geological triggers that led to the geochemical effects, followed by a range of biological processes.”